Touch screens of the prior art can take on five main forms. These five forms of touch screen input device include resistive, capacitive, surface acoustic wave (SAW), infrared (IR), and optical. Each of these types of touch screen has its own features, advantages and disadvantages.
Resistive is the most common type of touch screen technology. It is a low-cost solution found in many touch screen applications, including hand-held computers, PDA's, consumer electronics, and point-of-sale-applications. A resistive touch screen uses a controller and a specifically coated glass overlay on the display face to produce the touch. connection. The primary types of resistive overlays are 4-wire, 5-wire, and 8 wires. The 5-wire and 8-wire technologies are more expensive to manufacture and calibrate, while 4-wire provides lower image clarity. Two options are generally given: polished or anti-glare. Polished offers clarity of image, but generally introduces glare. Anti-glare will minimize glare, but will also further diffuse the light thereby reducing the clarity. One benefit of using a resistive display is that it can be accessed with a finger (gloved or not), pen, stylus, or a hard object. However, resistive displays are less effective in public environments due to the degradation in image clarity caused by the layers of resistive film, and its susceptibility to scratching. Despite the trade-offs, the resistive screen is the most popular technology because of its relatively low price (at smaller screen sizes), and ability to use a range of input means (fingers, gloves, hard and soft stylus).
Capacitive touch screens are all glass and designed for use in ATM's and similar kiosk type applications. A small current of electricity runs across the screen with circuits located at the corners of the screen to measure the capacitance of a person touching the overlay. Touching the screen interrupts the current and activates the software operating the kiosk. Because the glass and bezel that mounts it to the monitor can be sealed, the touch screen is both durable and resistant to water, dirt and dust. This makes it commonly used in harsher environments like gaming, vending retail displays, public kiosks and industrial applications. However, the capacitive touch screen is only activated by the touch of a human finger and a gloved finger, pen, stylus or hard object will not work. Hence, it is inappropriate for use in many applications, including medical and food preparation.
Surface acoustic wave (SAW) technology provides better image clarity because it uses pure glass construction. A SAW touch screen uses a glass display overlay. Sound waves are transmitted across the surface of the display. Each wave is spread across the screen by bouncing off reflector arrays along the edges of the overlay. Two receivers detect the waves. When the user touches the glass surface, the user's finger absorbs some of the energy of the acoustic wave and the controller circuitry measures the touch location. SAW touch screen technology is used in ATM's, Amusements Parks, Banking and Financial Applications and kiosks. The technology is not able to be gasket sealed, and hence is not suitable to many industrial or commercial applications. Compared to resistive and capacitive technologies, it provides superior image clarity, resolution, and higher light transmission.
Infrared technology relies on the interruption of an infrared light grid in front of the display screen. The touch frame or opto-matrix frame contains a row of infrared LEDs and photo transistors; each mounted on two opposite sides to create a grid of invisible infrared light. The frame assembly is comprised of printed wiring boards on which the opto-electronics are mounted and is concealed behind an infrared-transparent bezel. The bezel shields the opto-electronics from the operating environment while allowing the infrared beams to pass through. The infrared controller sequentially pulses the LEDs to create a grid of infrared light beams. When a stylus, such as a finger, enters the grid, it obstructs the beams. One or more phototransistors detect the absence of light and transmit a signal that identifies the x and y coordinates. Infrared touch screens are often used in manufacturing and medical applications because they can be completely sealed and operated using any number of hard or soft objects. The major issue with infrared is the “seating” of the touch frame is slightly above the screen. Consequently, it is susceptible to “early activation” before the finger or stylus has actually touched the screen. The cost to manufacture the infrared bezel is also quite high.
Optical imaging for touch screens uses a combination of line-scan cameras, digital signal processing, front or back illumination and algorithms to determine a point of touch. The imaging lenses image the user's finger, stylus or object by scanning along the surface of the display. This type of touch screen is susceptible to false readings due to moving shadows and bright lights and also requires that the screen be touched before a reading is taken. Attempts have been made to overcome these disadvantages. Touch screens using optical imaging technology are disclosed in the following publications.
A touch screen using digital ambient light sampling is disclosed in U.S. Pat. No. 4,943,806, in particular this patent discloses a touch input device that continuously samples and stores ambient light readings and compares these with previously taken readings. This is done to minimise the effect of bright light and shadows.
A touch screen for use with a computer system is disclosed in U.S. Pat. No. 5,914,709. In particular a user input device sensitive to touch is disclosed that uses threshold adjustment processing. A light intensity value is read and an “ON” threshold is established, this threshold measurement and adjustment is frequently and periodically performed.
This U.S. Pat. No. 5,317,140 patent discloses a method for optically determining the position and direction of an object on a touch screen display. In particular, a diffuser is positioned over the light sources to produce an average light intensity over the touch screen.
U.S. Pat. No. 5,698,845 discloses a touch screen display that uses an optical detection apparatus to modulate the ON/OFF frequency of light emitters at a frequency of twice the commercial AC line source. The receiver determines the presence of light and compares this to the actual signal transmitted.
U.S. Pat. No. 4,782,328 discloses a touch screen that uses a photosensor unit positioned at a predetermined height above the touch screen, and when a pointer nears the touch screen, rays of its reflected or shadowed ambient light allow it to be sensed.
U.S. Pat. No. 4,868,551 discloses a touch screen that can detect a pointer near the surface of the display by detecting light reflected by the pointer (reflected or diffusive).